Write an email to a publication/blog that has the same target audience as your startup.
One of the best ways to get a load of early adopters early is to get covered by a publication. Some publications will cover your story for you, while others will allow you to post as a guest author. Both types are awesome.
If you have a good controversial blog topic that you have written about or want to write about, do tip these publications to see if they want to pick it up. Again, a good place to search for popular bloggers blogging about your topic, use Technorati.
Also, if they have already covered your competition, it's more likely they'd be willing to post about your startup because it makes sense as a follow up entry.
Look at some of the pitching examples here:
See 2 cold-emailed pitches that got onto TechCrunch: http://viniciusvacanti.com/2011/03/28/two-cold-emailed-techcrunch-pitches-that-worked/